1. Field of the Invention
The present invention relates to electric double-layer capacitors, and particularly electric double-layer capacitors suitable for use as automotive power supplies and power supplies for electrical storage systems.
2. Prior Art
An electric double-layer capacitor is an electrochemical device composed of a positive electrode, a negative electrode and a solution containing positive and negative ions (electrolyte solution). It uses a very thin dielectric layer, or electric double layer, that forms at the interfaces between the electrodes and the electrolyte solution as a dielectric, and is capable of storing and discharging electricity by the adsorption and release of negative ions at the positive electrode and positive ions at the negative electrode.
In such an electric double-layer capacitor, a separator is generally provided between the positive electrode and the negative electrode for the dielectric separation of the positive and negative electrodes and to hold the electrolyte solution.
Examples of separators used in electric double-layer capacitors include separators made of cellulose fibers (see JP-A 1998-256088), polyolefin nonwoven fabrics (see JP-A 2001-68380, JP-A 2001-110678, and JP-A 2002-266281), and porous polymer films.
However, electric double-layer capacitors which use a cellulose separator do not have a long enough life on account of the low chemical stability of cellulose itself. Polyolefin nonwoven fabric and porous polymer film generally have a lower melting point than cellulose and thus lack thermal stability. In addition, these latter separators can be manufactured to a porosity of at best 40 to 60 vol %, which is low compared to the 60 to 70 vol % porosity of cellulose separators. Accordingly, the amount of electrolyte solution such separators can hold is small and they also have a poor ability to absorb electrolyte solution.
Separators composed of glass fibers are described in JP-A 1997-289140 and JP-A 1997-260214. These glass fiber separators have an excellent chemical stability, thermal stability and electrolyte absorption. In addition, because they can be manufactured to a porosity of about 80 vol %, they also have an excellent ability to hold electrolyte solution.
However, such glass fiber separators have a lower mechanical strength during drying and during electrolyte absorption than do the other types of separators mentioned above. Therefore, in elements which in fact contain a glass fiber separator, when the separator is subjected to pressure during capacitor production, the thickness decreases, making it difficult to maintain the shape of the separator. As a result, the porosity within the separator decreases.